From bf0d87583a842b9e8aaf2a9cd9dbc3e976df2af4 Mon Sep 17 00:00:00 2001 From: Colin Watson Date: Sun, 9 Feb 2014 16:09:58 +0000 Subject: Allow harmless group-writability Allow secure files (~/.ssh/config, ~/.ssh/authorized_keys, etc.) to be group-writable, provided that the group in question contains only the file's owner. Rejected upstream for IMO incorrect reasons (e.g. a misunderstanding about the contents of gr->gr_mem). Given that per-user groups and umask 002 are the default setup in Debian (for good reasons - this makes operating in setgid directories with other groups much easier), we need to permit this by default. Bug: https://bugzilla.mindrot.org/show_bug.cgi?id=1060 Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=314347 Last-Update: 2013-09-14 Patch-Name: user-group-modes.patch --- auth-rhosts.c | 6 ++---- auth.c | 9 +++----- misc.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++- misc.h | 2 ++ platform.c | 16 -------------- readconf.c | 5 +++-- ssh.1 | 2 ++ ssh_config.5 | 2 ++ 8 files changed, 82 insertions(+), 29 deletions(-) diff --git a/auth-rhosts.c b/auth-rhosts.c index ee9e827..2ff2cff 100644 --- a/auth-rhosts.c +++ b/auth-rhosts.c @@ -271,8 +271,7 @@ auth_rhosts2_raw(struct passwd *pw, const char *client_user, const char *hostnam return 0; } if (options.strict_modes && - ((st.st_uid != 0 && st.st_uid != pw->pw_uid) || - (st.st_mode & 022) != 0)) { + !secure_permissions(&st, pw->pw_uid)) { logit("Rhosts authentication refused for %.100s: " "bad ownership or modes for home directory.", pw->pw_name); auth_debug_add("Rhosts authentication refused for %.100s: " @@ -298,8 +297,7 @@ auth_rhosts2_raw(struct passwd *pw, const char *client_user, const char *hostnam * allowing access to their account by anyone. */ if (options.strict_modes && - ((st.st_uid != 0 && st.st_uid != pw->pw_uid) || - (st.st_mode & 022) != 0)) { + !secure_permissions(&st, pw->pw_uid)) { logit("Rhosts authentication refused for %.100s: bad modes for %.200s", pw->pw_name, buf); auth_debug_add("Bad file modes for %.200s", buf); diff --git a/auth.c b/auth.c index bd6a026..782b7f8 100644 --- a/auth.c +++ b/auth.c @@ -425,8 +425,7 @@ check_key_in_hostfiles(struct passwd *pw, Key *key, const char *host, user_hostfile = tilde_expand_filename(userfile, pw->pw_uid); if (options.strict_modes && (stat(user_hostfile, &st) == 0) && - ((st.st_uid != 0 && st.st_uid != pw->pw_uid) || - (st.st_mode & 022) != 0)) { + !secure_permissions(&st, pw->pw_uid)) { logit("Authentication refused for %.100s: " "bad owner or modes for %.200s", pw->pw_name, user_hostfile); @@ -488,8 +487,7 @@ auth_secure_path(const char *name, struct stat *stp, const char *pw_dir, snprintf(err, errlen, "%s is not a regular file", buf); return -1; } - if ((!platform_sys_dir_uid(stp->st_uid) && stp->st_uid != uid) || - (stp->st_mode & 022) != 0) { + if (!secure_permissions(stp, uid)) { snprintf(err, errlen, "bad ownership or modes for file %s", buf); return -1; @@ -504,8 +502,7 @@ auth_secure_path(const char *name, struct stat *stp, const char *pw_dir, strlcpy(buf, cp, sizeof(buf)); if (stat(buf, &st) < 0 || - (!platform_sys_dir_uid(st.st_uid) && st.st_uid != uid) || - (st.st_mode & 022) != 0) { + !secure_permissions(&st, uid)) { snprintf(err, errlen, "bad ownership or modes for directory %s", buf); return -1; diff --git a/misc.c b/misc.c index de7e1fa..5704fa6 100644 --- a/misc.c +++ b/misc.c @@ -51,8 +51,9 @@ #include #ifdef HAVE_PATHS_H # include -#include #endif +#include +#include #ifdef SSH_TUN_OPENBSD #include #endif @@ -61,6 +62,7 @@ #include "misc.h" #include "log.h" #include "ssh.h" +#include "platform.h" /* remove newline at end of string */ char * @@ -647,6 +649,71 @@ read_keyfile_line(FILE *f, const char *filename, char *buf, size_t bufsz, return -1; } +/* + * return 1 if the specified uid is a uid that may own a system directory + * otherwise 0. + */ +int +platform_sys_dir_uid(uid_t uid) +{ + if (uid == 0) + return 1; +#ifdef PLATFORM_SYS_DIR_UID + if (uid == PLATFORM_SYS_DIR_UID) + return 1; +#endif + return 0; +} + +int +secure_permissions(struct stat *st, uid_t uid) +{ + if (!platform_sys_dir_uid(st->st_uid) && st->st_uid != uid) + return 0; + if ((st->st_mode & 002) != 0) + return 0; + if ((st->st_mode & 020) != 0) { + /* If the file is group-writable, the group in question must + * have exactly one member, namely the file's owner. + * (Zero-member groups are typically used by setgid + * binaries, and are unlikely to be suitable.) + */ + struct passwd *pw; + struct group *gr; + int members = 0; + + gr = getgrgid(st->st_gid); + if (!gr) + return 0; + + /* Check primary group memberships. */ + while ((pw = getpwent()) != NULL) { + if (pw->pw_gid == gr->gr_gid) { + ++members; + if (pw->pw_uid != uid) + return 0; + } + } + endpwent(); + + pw = getpwuid(st->st_uid); + if (!pw) + return 0; + + /* Check supplementary group memberships. */ + if (gr->gr_mem[0]) { + ++members; + if (strcmp(pw->pw_name, gr->gr_mem[0]) || + gr->gr_mem[1]) + return 0; + } + + if (!members) + return 0; + } + return 1; +} + int tun_open(int tun, int mode) { diff --git a/misc.h b/misc.h index 374c33c..89e1f75 100644 --- a/misc.h +++ b/misc.h @@ -135,4 +135,6 @@ char *read_passphrase(const char *, int); int ask_permission(const char *, ...) __attribute__((format(printf, 1, 2))); int read_keyfile_line(FILE *, const char *, char *, size_t, u_long *); +int secure_permissions(struct stat *st, uid_t uid); + #endif /* _MISC_H */ diff --git a/platform.c b/platform.c index f35ec39..9a23e6e 100644 --- a/platform.c +++ b/platform.c @@ -197,19 +197,3 @@ platform_krb5_get_principal_name(const char *pw_name) return NULL; #endif } - -/* - * return 1 if the specified uid is a uid that may own a system directory - * otherwise 0. - */ -int -platform_sys_dir_uid(uid_t uid) -{ - if (uid == 0) - return 1; -#ifdef PLATFORM_SYS_DIR_UID - if (uid == PLATFORM_SYS_DIR_UID) - return 1; -#endif - return 0; -} diff --git a/readconf.c b/readconf.c index fde6b41..cc1a633 100644 --- a/readconf.c +++ b/readconf.c @@ -39,6 +39,8 @@ #include #include #include +#include +#include #ifdef HAVE_UTIL_H #include #endif @@ -1626,8 +1628,7 @@ read_config_file(const char *filename, struct passwd *pw, const char *host, if (fstat(fileno(f), &sb) == -1) fatal("fstat %s: %s", filename, strerror(errno)); - if (((sb.st_uid != 0 && sb.st_uid != getuid()) || - (sb.st_mode & 022) != 0)) + if (!secure_permissions(&sb, getuid())) fatal("Bad owner or permissions on %s", filename); } diff --git a/ssh.1 b/ssh.1 index cc53343..feb0e89 100644 --- a/ssh.1 +++ b/ssh.1 @@ -1459,6 +1459,8 @@ The file format and configuration options are described in .Xr ssh_config 5 . Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not writable by others. +It may be group-writable provided that the group in question contains only +the user. .Pp .It Pa ~/.ssh/environment Contains additional definitions for environment variables; see diff --git a/ssh_config.5 b/ssh_config.5 index bbf638b..ab8f271 100644 --- a/ssh_config.5 +++ b/ssh_config.5 @@ -1830,6 +1830,8 @@ The format of this file is described above. This file is used by the SSH client. Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not accessible by others. +It may be group-writable provided that the group in question contains only +the user. .It Pa /etc/ssh/ssh_config Systemwide configuration file. This file provides defaults for those